The paper can be found at: Tor Helleseth, Daniel J. Katz, and Chunlei Li, The resolution of Niho's last conjecture concerning sequences, codes, and Boolean functions, IEEE Transactions on Information Theory, 67(10): 6952-6962 (2021).
A preprint version is available at: arXiv: 2006.12239 [math.NT]
The work of Katz on this paper and the accompanying materials here was supported in part by the National Science Foundation under Grants 1500856 and 1815487.
Any opinions, findings, and conclusions or recommendations expressed in this material are those of the authors and do not necessarily reflect the views of the National Science Foundation.
If you are interested in using any code or data presented here for your research, please contact me at email: [my first name] [dot] [my last name] [at] csun [dot] edu
The Sage code used to produce decompositon in Lemma 5.2 of the paper is called verification-for-Lemma-5.2.sage and can be found in the program directory.